Conventionally, there have been proposed light source lighting circuits each which receive electrical power from a DC power supply to light a light source. Each light source lighting circuit includes a backflow preventer that protects a circuit when the DC power supply is connected to the circuit with reverse polarity, and a current controller that supplies a driving current to the light source (see, for example, JP 2012-38496 A, hereinafter referred to as “Document 1”).
The backflow preventer in the light source lighting circuit of Document 1 includes a first control terminal and a second control terminal each which are connected to the DC power supply via a switch. The current controller includes a common terminal that is connected to the first control terminal and the second control terminal, and a switching terminal that is connected to the first control terminal. The current controller operates with a DC power supply voltage applied to the common terminal to supply the driving current to the light source. Also, the current controller changes the driving current supplied to the light source according to whether or not a voltage is applied to the switching terminal, that is, whether or not the DC power supply voltage is applied to the first control terminal, and changes brightness of the light source according to the changed driving current.
Here, the backflow preventer includes a first P-channel FET connected between the first control terminal and the common terminal, and a second P-channel FET and a diode connected in series between the second control terminal and the common terminal.
When the DC power supply voltage is applied to the first control terminal, the first P-channel FET becomes in an on-state, and the second P-channel FET becomes in an off-state, and the DC power supply voltage is applied to the current controller via the first P-channel FET. When the DC power supply voltage is not applied to the first control terminal and the DC power supply voltage is applied to the second control terminal, the second P-channel FET becomes in the on-state, and the DC power supply voltage is applied to the current controller via the second P-channel FET and the diode. Also, when the DC power supply voltage with reverse polarity is applied to the first control terminal or the second control terminal, the first P-channel FET and the second P-channel FET are in the off-state, and the current controller is protected by the diode.
In the light source lighting circuit described in Document 1, when the DC power supply voltage is not applied to the first control terminal and the DC power supply voltage is applied to the second control terminal, the DC power supply voltage is applied to the current controller via the second P-channel FET and the diode. Because a forward voltage of the diode is generally higher than an on-voltage of the P-channel FET, when the DC power supply voltage is not applied to the first control terminal and the DC power supply voltage is applied to the second control terminal, a large loss occurs compared with the case where the DC power supply voltage is applied to the first control terminal.